Feature: If no shipping required redirect directly to Payment

I hope this PR looks better. It has just two commits, and the code is cleaned. 
The two commits are:
          
               21e7ea3 add functional testing for the new redirects
              oscar/apps/checkout/fixtures/no_shipping.json
              tests/config.py
              tests/functional/checkout_tests.py

And:
            
               cee7ee5 redirect to checkout if no shipping required
               oscar/apps/checkout/views.py

The later commit (cee7ee5) is the actual code change in `IndexView` of the checkout application. The previous commit (21e7ea3) contains a minor change to the configuration file of the tests. It is needed to run the tests.
The fixtures include just one course without a shipping method. In  `tests/functional/checkout_tests.py` there is a new class testing the new functionality.